In recent years there have been advances in the development of micrometer scale x-ray sources, such that it is now possible to produce a plurality of x-ray sources with a typical distance between the x-ray sources of the order of 100 μm to 1 cm or more.
An example of such a two-dimensional x-ray source is provided in WO 2011/017645“apparatus for producing x-rays for use in imaging”.
Known two-dimensional x-ray sources as in WO 2011/017645 have all of their sources on simultaneously, i.e. at the point of initiating the x-ray emission field emission the surface electrons will occur at each of the field emitters and x-ray photons (bremsstrahlung) will be emitted simultaneously from multiple sites as electrons strike the target material.
For certain x-ray imaging modalities it may be desirable to be able to control the sequence of the activation of individual x-ray sources within a plurality of x-ray sources. For example, it may be advantageous to activate the x-ray sources in a sequential and row by row manner known as raster scanning which is used in many electronic imaging devices.
A well known problem in electron and x-ray generation is the control of the electron current and hence of the resulting x-ray flux. In conventional systems, intermediate voltage grids are sometimes used to blank the electron emission. In other cases, high voltage switches are used to shut off the potential. In still other configurations, such as those found in Spindt arrays, transistors control the moderate-voltage supplied to each of the field-enhanced emitters and a further stage is used to increase the voltage (energy) of the electrons to the final end-point energy desired from the x-ray source.
Extending these methods to large arrays can prove impractical and costly. Grids can limit the emitter density and introduce a source of arcing and breakdown. Transistor arrays can lead to complex switching controls and the low voltages of emission can lead to reduced current due to space charge effects. Therefore, it is desirable to invent a new mechanism of controlling the production of x-rays from arrays of electron emitters; one which does not rely on high voltage switching.